I had expected that Monaco's best-known twentieth-century resident would win here, but in fact there is a clear lead for yet another crime novel, published in 2002 by a writer who was much better known in his native Italy as a comedian and actor; it seems to have been a successful if temporary shift of genre for him. It is:

Io uccido / I Kill, by Giorgio Faletti

Another best-selling crime novel, a 2012 collaboration between two well-known American authors, seems to be mainly set in France although the crime itself takes place on a boat off Monte Carlo:

Guilty Wives, by James Patterson and David Ellis

I'm happy with my Italian crime novel as today's winner, but here are a couple more. First, a short novel from 1954 by one of England's best-known twentieth century writers, a moral tale of winning at the casino failing to bring one happiness:

Loser Takes All, by Graham Greene

And second, a 2011 novel about the life of the chef Auguste Escoffier, torn between his lover Sarah Bernhardt and his wife in Monte Carlo, where (judging by a quick skim of online reviews) a bit more than half of the book is set:

White Truffles in Winter, by N.M. Kelby

And the best-known biography of the principality's best-known resident, tying in with the recent widely-panned biopic (according to the Guardian, "so awe-inspiringly wooden that it is basically a fire-risk"), is:

Once Upon a Time: Behind the Fairy Tale of Princess Grace and Prince Rainier, by J. Randy Taraborrelli